Since the demonstration of the first organic solar cell with an efficiency in the percent range by Tang, et al. in 1986 [C. W. Tang, et al. Appl. Phys. Lett. 48, 183 (1986)], organic materials have been studied intensively for various electronic and optoelectronic components. Organic solar cells consist of a sequence of thin layers (typically 1 nm to 1 μm) of organic materials, which are preferably vapor-deposited under reduced pressure or spun on from a solution. The electrical contacts can be established by metal layers, transparent conductive oxides (TCOs) and/or transparent conductive polymers (PEDOT-PSS, PANI).
A solar cell converts light energy to electrical energy. The term “photoactive” here likewise signifies the conversion of light energy to electrical energy. In contrast to inorganic solar cells, the light does not directly generate free charge carriers in organic solar cells; instead, excitons are formed at first, i.e., electrically neutral excitation states (bound electron-hole pairs). Only in a second step are these excitons separated into free charge carriers, which then contribute to electrical current flow.
The advantage of such organic-based components over the conventional inorganic-based components (semiconductors such as silicon, gallium arsenide) is that the optical absorption coefficients are in some cases extremely high (up to 2×105 cm-1), and so it is possible to produce very thin solar cells with low material consumption and energy expenditure. Further technological aspects are low costs, the possibility of producing flexible large-area components on plastic films, and the virtually unlimited possible variations and the unlimited availability of organic chemistry.
One possible implementation of an organic solar cell which has already been proposed in the literature is that of a pin diode [Martin Pfeiffer, “Controlled doping of organic vacuum deposited dye layers: basics and applications,” PhD thesis TU-Dresden, 1999] with the following layer structure:
0. carrier, substrate,
1. base contact, usually transparent,
2. p layer(s),
3. i layer(s),
4. n layer(s),
5. top contact.
In this context, n and p mean n- and p-doping respectively, this leading to an increase in the density of, respectively, free electrons and holes in the thermal equilibrium state. However, it is also possible that the n layer(s) or p layer(s) are at least partially nominally undoped and have preferentially n-conducting or preferentially p-conducting properties solely due to the material properties (for example different mobilities), due to unknown impurities (for example remaining residues from the synthesis, decomposition or reaction products during layer production) or due to environmental influences (for example adjoining layers, inward diffusion of metals or other organic materials, gas doping from the ambient atmosphere). In this context, such layers should be understood primarily to be transport layers. The term “i layer,” in contrast, denotes a nominally undoped layer (intrinsic layer). In this context, one or more i layers may layers consist either of one material or of a mixture of two materials (called interpenetrating networks or a bulk heterojunction; M. Hiramoto, et al. Mol. Cryst. Liq. Cryst., 2006, 444, pp. 33-40). The light incident through the transparent base contact produces excitons (bound electron-hole pairs) in the i layer or in the n/p layer. These excitons can be separated only by very high electrical fields or at suitable interfaces. In organic solar cells, sufficiently high fields are not available, and so all promising concepts for organic solar cells are based on exciton separation at photoactive interfaces. As a result of diffusion, the excitons arrive at such an active interface, where electrons and holes are separated from one another. The material which absorbs the electrons is referred to as the acceptor, and the material which absorbs the hole as the donor. The separating interface may be between the p (n) layer and the i layer, or between two i layers. In the installed electrical field of the solar cell, the electrodes are then transported away to the n region and the holes to the p region. The transport layers are preferably transparent or substantially transparent materials with a wide band gap, as described, for example, in WO 2004/083958. Wide-gap materials refer here to materials whose absorption maximum is in the wavelength range of <450 nm, preferably at <400 nm.
Since the light always first produces excitons and no free charge carriers as yet, the low-recombination diffusion of excitons to the active interface plays a critical role in organic solar cells. In order to make a contribution to the photocurrent, the exciton diffusion length in a good organic solar cell must therefore significantly exceed the typical penetration depth of light in order that the predominant portion of the light can be utilized. Thin layers or organic crystals perfect in terms of structure and with regard to chemical purity do indeed meet this criterion. For large-area applications, however, the use of monocrystalline organic materials is impossible and the production of multiple layers with sufficient structural perfection is still very difficult to date.
If the i layer is a mixed layer, the task of light absorption is assumed either by only one of the components or else by both. The advantage of mixed layers is that the excitons produced have to cover only a very short distance before arriving at the domain boundary where they are separated. The electrons or holes are transported away separately in the respective materials. Since the materials are in contact with one another throughout the mixed layer, it is crucial in this concept that the separate charges have a long lifetime on the respective material and continuous percolation pathways exist for both charge carrier types from any site toward the respective contact.
U.S. Pat. No. 5,093,698 discloses the doping of organic materials. Addition of an acceptor-type or donor-type doping substance increases the equilibrium charge carrier concentration in the layer and enhances the conductivity. According to U.S. Pat. No. 5,093,698, the doped layers are used as injection layers at the interface to the contact materials in electroluminescent components. Similar doping approaches are also appropriate analogously for solar cells.
The literature discloses various possible ways of implementing the photoactive i layer. For instance, it may be a double layer (EP Patent Document 0000829) or a mixed layer (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991)). Also known is a combination of double and mixed layers (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991); U.S. Pat. No. 6,559,375). It is likewise known that the mixing ratio is different in different regions of the mixed layer (U.S. Patent Publication No. 2005/0110005), and that the mixing ratio has a gradient.
Also known from the literature are tandem or multiple solar cells (Hiramoto, Chem. Lett., 1990, 327 (1990); DE 102004014046).
Additionally known from the literature are organic pin tandem cells (DE Patent Publication 102004014046): the structure of such a tandem cell consists of two individual pin cells, the layer sequence “pin” describing the sequence of a p-doped layer system, an undoped photoactive layer system and an n-doped layer system. The doped layer systems preferably consist of transparent materials, called wide-gap materials/layers, and in this case they may also be partly or wholly undoped or else have different dopant concentrations as a function of location, or have a continuous gradient in the dopant concentration. Specifically, regions with very low doping or high doping are also possible in the boundary region to the electrodes, in the boundary region to another doped or undoped transport layer, in the boundary region to the active layers or, in the case of tandem or multiple cells, in the boundary region to the adjoining pin or nip subcell, i.e., in the region of the recombination zone. Any desired combination of all these features is also possible. Of course, such a tandem cell may also be what is called an inverted structure (e.g., nip tandem cell). All these possible tandem cell implementation forms are referred to hereinafter by the term “pin tandem cells.”
In the context of the present invention, small molecules are understood to mean nonpolymeric organic molecules with monodisperse molar masses between 100 and 2000, which are in the solid phase under standard pressure (air pressure of the ambient atmosphere) and at room temperature. More particularly, these small molecules may also be photoactive, “photoactive” being understood to mean that the molecules change their charge state under incidence of light.
The problem with organic solar cells is currently that even the highest efficiencies of 6-7% achieved to date in the laboratory are still too low. For most applications, specifically large-area applications, an efficiency of approx. 10% is considered to be necessary. Due to the relative poor transport properties of organic semiconductors (as compared with inorganic semiconductors) and the associated limitation in the layer thicknesses of the absorbers usable in organic solar cells, it is generally assumed that such efficiencies can best be implemented with the aid of tandem cells (Tayebeh Ameri, et al., Organic tandem solar cells: A review, Energy Environ. Sci., 2009, 2, 347-363; DE 102004014046.4). Specifically, efficiencies up to 15% will probably be possible in future only with the aid of tandem cells. In the conventional construction of such a tandem solar cell, various absorber systems are used in the two subcells, these absorbing in different parts (possibly even overlapping) of the solar spectrum in order to exploit a maximum range. The absorber system of one subcell absorbs here in the shorter-wave spectral range (preferably in the visible range) and the absorber system of the other subcell in the longer-wave spectral range (preferably in the infrared range). FIG. 1 shows the schematic distribution of the absorption spectra in the two subcells of a tandem cell of conventional construction.
The disadvantage of these tandem cells is that the subcell with the infrared absorber delivers a lower open-circuit voltage than the other subcell, and this subcell can thus make only a relatively small contribution to the efficiency of the component.